High-strength steel sheet and manufacturing method thereof

Abstract

Provided are a high-strength steel sheet and a method for manufacturing same, the high-strength steel sheet including: an alloy system having C, Si, Mn, Cr, Al, Nb, Ti, B, P, S, N, and the remainder of Fe and other inevitable impurities. The contents of C, Si, and Al satisfy equation (1) below. The microstructure includes, by an area fraction, greater than 50% to 70% or less of tempered martensite, and the remainder of residual austenite, fresh martensite, ferrite, and bainite, in which a cementite phase as a second phase is precipitated and distributed in an area fraction of 1-3% between bainite laths, or at a lath on the tempered martensite or grain boundaries. [Equation (1)][C]+([Si]+[Al])/5; 0.35 wt. % (wherein [C], [Si], and [Al] denote wt % of C, Si, and Al, respectively.)

The invention claimed is: 
     
         1 . A high-strength steel sheet comprising:
 by weight percent (wt %), 0.12% to less than 0.17% of carbon (C), 0.3% to 0.8% of silicon (Si), 2.5% to 3.0% of manganese (Mn), 0.4% to 1.1% of chromium (Cr), 0.01% to 0.3% of aluminum (Al), 0.01% to 0.03% of niobium (Nb), 0.01% to 0.03% of titanium (Ti), 0.001% to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S): 0.01% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities,   wherein the contents of C, Si, and Al satisfy mathematical equation (1) below,   a microstructure of the high-strength steel sheet includes, by area fraction, more than 50% to 70% or less of tempered martensite and remaining retained austenite, fresh martensite, ferrite and bainite, and   wherein a cementite phase as a second phase is precipitated and distributed in an area fraction of 1% or more and 3% or less between the bainite laths or at the laths or grain boundaries of the tempered martensite phase,   
       
         
           
             
               
                 
                   
                     
                       
                         [ 
                         C 
                         ] 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               [ 
                               Si 
                               ] 
                             
                             + 
                             
                               [ 
                               Al 
                               ] 
                             
                           
                           ) 
                         
                         / 
                         5 
                       
                     
                     ≤ 
                     
                       0.35 
                         
                       
                         wt 
                         . 
                             
                         % 
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Equation 
                       ⁢ 
                           
                       
                         ( 
                         1 
                         ) 
                       
                     
                     ] 
                   
                 
               
             
           
         
         wherein [C], [Si] and [Al] refer to the wt % of C, Si and Al, respectively. 
       
     
     
         2 . The high-strength steel sheet of  claim 1 , wherein the high-strength steel sheet includes more than 1% and less than 4% of the retained austenite, more than 10% and less than 20% of the fresh martensite, and more than 0% of the ferrite to less than 5%, and the balance is bainite. 
     
     
         3 . The high-strength steel sheet of  claim 1 , wherein, when the micro Vickers hardness test is performed, a difference between a 25%-th hardness value and a 75%-th hardness value may be distributed in a range between 100 and 150. 
     
     
         4 . The high-strength steel sheet of  claim 1 , wherein the steel sheet further includes, by wt %, one or more of 0.1% or less of copper (Cu), 0.1% or less of nitrogen (Ni), 0.3% or less of molybdenum (Mo), and 0.03% or less of vanadium (V). 
     
     
         5 . The high-strength steel sheet of  claim 1 , wherein the steel sheet has a tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, a yield ratio of 0.65 to 0.85, a hole expansion ratio (HER) of 25% or more, and an elongation of 7 to 14%. 
     
     
         6 . The high-strength steel sheet of  claim 1 , wherein the steel sheet is a cold rolled steel sheet. 
     
     
         7 . The high-strength steel sheet of  claim 1 , wherein a hot-dip galvanized layer is formed on at least one surface of the steel sheet. 
     
     
         8 . The high-strength steel sheet of  claim 1 , wherein an alloying hot-dip galvanized layer is formed on at least one surface of the steel sheet. 
     
     
         9 . A method of manufacturing a high-strength steel sheet of  claim 1 , the method comprising:
 preparing a slab comprising, by wt % ,0.12% to less than 0.17% of carbon (C), 0.3% to 0.8% of silicon (Si), 2.5% to 3.0% of manganese (Mn), 0.4% to 1.1% of chromium (Cr), 0.01% to 0.3% of aluminum (Al), 0.01% to 0.03% of niobium (Nb), 0.01% to 0.03% of titanium (Ti), 0.001% to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S): 0.01% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities, wherein the contents of C, Si, and Al satisfy Equation 1 below;   heating the slab to a temperature range of 1150° C. to 1250° C.;   finish hot rolling the heated slab within a temperature range of finish delivery temperature (FDT) of 900° C. to 980° C. to obtain a hot rolled steel sheet;   cooling the hot rolled steel sheet at an average cooling rate of 10° C./sec to 100° C./sec to obtain a cooled steel sheet;   coiling the cooled steel sheet in a temperature range of 500° C. to 700° C.;   cold rolling the coiled steel sheet at a cold-rolling reduction ratio of 30% to 60% to obtain a cold rolled steel sheet;   continuously annealing the cold rolled steel sheet in a temperature range of (Ac3+30° C.˜Ac3+80° C.);   primarily cooling the continuously annealed steel sheet at an average cooling rate of 10° C./s or less to a temperature range of 500° C. to 700° C. and secondarily cooling the primarily cooled steel sheet at an average cooling rate of 10° C./s or more to a temperature range of 280° C. to 380° C.; and   reheating the secondarily cooled steel sheet at a temperature increase rate of 5° C./s or less to a temperature range of 380° C. to 480° C.,   
       
         
           
             
               
                 
                   
                     
                       
                         [ 
                         C 
                         ] 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               [ 
                               Si 
                               ] 
                             
                             + 
                             
                               [ 
                               Al 
                               ] 
                             
                           
                           ) 
                         
                         / 
                         5 
                       
                     
                     ≤ 
                     
                       0.35 
                           
                       wt 
                       ⁢ 
                           
                       % 
                     
                   
                 
                 
                   
                     [ 
                     
                       Equation 
                       ⁢ 
                           
                       
                         ( 
                         1 
                         ) 
                       
                     
                     ] 
                   
                 
               
             
           
         
         wherein [C], [Si] and [Al] refer to the wt % of C, Si and Al, respectively. 
       
     
     
         10 . The method of  claim 9 , wherein the slab further includes, by wt %, 0.1% or less of copper (Cu), 0.1% or less of nickel (Ni), 0.3% or less of molybdenum (Mo), and 0.03% or less of vanadium (V). 
     
     
         11 . The method of  claim 9 , further comprising performing hot dip galvanizing at a temperature range of 480° C. to 540° C., after the reheating. 
     
     
         12 . The method of  claim 11 , wherein, after the performing of the hot-dip galvanizing, an alloying heat treatment is performed and a cooling is subsequently performed to room temperature. 
     
     
         13 . The method of  claim 11 , wherein, after the cooling to room temperature, a temper rolling of less than 1% is performed.